Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~7~ 7
_ACKGROUND OF THE INVENTION
This invention relates to the manufacture of paper
and cardboard.
Paper and cardboard are generally made by pouring an
aqueous suspension of cellulosic fibres in the form of a pulp
~n to a wire mesh screen formed from a metal or a synthetic
plastics material, and removing the water by drainage and/or
other means such as suction, pressing and thermal evaporation.
The cellulosic fibres are generally derived from wood which
ha5 been mechanically and chemically treated to orm a pulp
o fibrillated ibres which, when deposited on the wire mesh
screen used for forming the paper or cardboard, interlock to
form a web. Other sources of cellulosic fibres include sisal,
esparto, hemp, jute, straw, bagasse, cotton linters and rags.
The addition of a white filler to the cellulosic
fibres improves the opacity, whiteness and ink receptivity of
paper or cardboard which is formed from the fibres. The
fillex is also cheaper than ~he cellulosic fibres and there-
; ore replacing some of the cellulosic fibres with the filler
can result in a cheaper product. The white filler may be,
for example, kaolin, calcium sulphate, calcium carbonate,
talc, silica or a synthetic silicate. However the use of a
filler has the following disadvantages: (a) when the filler
c~ntains relatively coarse particles, i. e. particles having
a diameter larger than about 10 ~m equivalent spherical
diameter, o a hard mineral the paper or cardboard product
tends to become abrasive with consequent wear o type face
and printing machinery, and
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467
(b) when the filler contains a high proportion of relatively
fine particles, i.e. particles having a diameter smaller
than about 2 ,um equivalent spherical diameter, the strength
of the paper or cardboard product is reduced and in
addition unless expensive retention aids are used a pro-
portion of the filler which is added to the cellulosic
fibres tends not to be retained in the web of fibres but
escapes with the "white water", i.e. the water which
drains through the web and through the mesh screen, thus
introducing the problem of recovering the mineral particles
before the effluent water can be discharged.
Many materials, including aluminium sulphate,
starch and starch derivatives, have been incorporated
in the pulp of filler and cellulosic fibres with a view
5 to binding the filler to the cellulosic fibres.
SUMMARY OF THE INVENTION
According to the present: invention there is
provided a method of manufacturi.ng paper or cardboard
which method comprises the steps of mixing an aqueous
solution or dispersion of a cationic starch with an
aqueous suspension of a kaolinitic clay filler to form a
mixture containing flocs of starch and clay filler;
thereafter adding the mixture thus obtained to an aqueous
stock of cellulosic fibres to form a furnish containing
~5 the flocs of starch and kaolinitic clay filler, and the
cellulosic fibres; and then forming the furnish into
paper or cardboard, the product of the rate at which shear
is applied to, and the period for which shear is applied
to, said flocs during the formation~of said mixture and
said furnish being such that the flocs are reduced in size
sufficiently to enable substantially all of them to pass
through a No. 200 mesh British Standard sieve (nominal
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,.~ ,
~097467
aperture 76 ,um) but not so much that substantially all
of them can pass through a No. 300 mesh British Standard
sieve (nominal aperture 53 ,~m).
The cationic starch carries positive charges
which improve bonding to the cellulosic fibr~s. Preferably,
the cationic starch carries primary, secondary or tertiary
amino groups or quarternary ammonium groups. The degree
of cationicity (generally expressed in terms of the nitrogen
; content of the starch) is important; and usually the starchemployed should have a nitrogen content in the range of ;~
from 0.03~ to 1.0% by weight, with starches having a nitrogen
content between 0.1 and 0.25% by weight being particularly
effective. It also appears that as the molecular weight
,of the starch is increased so the effect on the strength
of the paper is improved, although the viscosity of a
suspension of the starch increases.
` The quantity of cationic starch used will
generally be in the range from about 1% to about 20% by
weight, preferably from 2% to 10% by weight, based on
the weight of dry kaolinitic clay filleri and there wilI
generally be present in the paper or cardboard from about
0.5 to about 5.0 g of cationic starch, preferably from
l to 3.5 g. of cationic starch per lO0 g. of dry furnish,
i.e. cellulosic fibres and clay filler.
A further improvement in strength may also be
achieved if both the aqueous suspension of cellulosic
fibres and the aqueous suspension of kaolinitic clay filler
are tre,ated with the cationic starch before they are mixed
together. The total amount of cationic starch used will
again generally be in the range of from 0.5 g to 5.0 g of
starch per 100 g of dry furnish.
The strength of the paper or cardboard which is
t~
~L~"7~67
formed from the mixture of kaolinitic clay filler,
cationic starch and cellulosic fibres is increased if the
filler is substantially free of particles having an
equivalent spherical diameter smaller than 1 /um. Generally,
the filler should contain not more than 18% by weight,
and preferably not more than 15~ by weight, of particles
having an equivalent spherical diameter smaller than 2 lum,
and not more than 10% by weight of particles having an
equivalent spherical diameter smaller than 1 ,um.
On mixing an aqueous solution or dispersion of
a cationic starch with an aqueous suspension of a kaolinitic
clay filler the particles of filler are flocculated and
bound to each other in such a way that the flocs are them-
selves subsequently bound to the cellulosic fibres. In
order to obtain the highest stren~th in a paper manufactured
according to the method of the invention, it is important
that the amount of shear to which the mixture of the kao-
linitic clay filler and cationic starch is exposed should
be moderate, i.e. neither too little nor too great. By
the "amount of shear" there is meant herein the product of
the rate at which shear is applied to the flocs and the
period for which the shear is applied to the flocs. The
amount of shear to which the mixture of kaolinitic clay
filler and cationic starch is exposed should be at least
that which is required to break down the floc structure
until substantially all of the starch/filler mixture can
pass through a No. 200 mesh British Standard sieve (nominal
aperture 76 ,um) but should not be so great that the floc
structure is broken down to the extent that the particle
size of the flocs of starch and clay filler is substantially
the same as that of the untreated filler and can all pass
through a No. 300 mesh British Standard sieve (nominal
,. ,
'~
.. ..
7~67
aperture 53 ~m). If the floc structure is not broken down
to the extent noted above a paper containing the filler is
unacceptable because of lumps of undispersed filler and, on
the other hand, if the flock structure is broken down too much
the treated filler w~uld give no improvement in the strength of the
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,~1 .
~L~97467
filled paper as compared with an untreated filler. The
a~ount of shear to which the mixture of kaolinitic clay
filler and cationic starch is exposed is important not
only in ~he operation of mixing the starch with the clay
filler but also in subsequent operations such as that of
mixing the flocs of starch and clay filler with the cellu-
losic fibres to form the furnish.
The invention is illustrated by the following
Examples.
EXAMæLE 1
For the experiments described in this Example
the apparatus shown schematically in the accompanying
drawing was employed.
A. An aqueous suspension containing 2~ by weight
of cellulosic fibres (obtained by beating and reEining a
bleached sulphite pulp) was mixed in a stirred tank 1 with
1.5% by weight, based on the weight of dry cellulosic
fibres, of fortified rosin size and 3.0% by weight of
powdered aluminium sulphate. The resulting suspension of
sized fibres was delivered by a pump 2 through a conduit 3
to a constant head tank 4 from which the overflow returned
to tank 1 through a conduit 5. Clean water was supplied
via a conduit 16 to a second constant head tank 6 from
which the overflow passed through a conduit 7 to a reser-
voir (not shown).
The suspension of sized fibres flowed from tank4 through a conduit 8 and water from tank 6 through a
conduit 9 to a tank 10 where they were mixed in the propor-
tions 3 parts by weight of water to 1 part by weight of
suspension to dilute the suspension to 0.5% by weight of
cellulosic fibres.
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~1~9~4~
In a stirred tank 11 there were mixed together
water, a china clay filler in a flocculatecl state and a
cationic starchcon-taining tertiary amine groups. The
china clay had a particle size distribution such that 25% -
by weight consisted of particles having an equivalent
spherical diameter larger than 10~m and 20% by weight
consisted of particles having an equivalent spherical
diameter smaller than 2~m. The starch was added in the
proportion of 5% by weight, based on the weight of dry
clay. The flocculated mixture of clay and starch was
run through a conduit 12 to the tank 10 and was mixed
with the s~e ~ 6r~r of sized fibres in di:F:Eeren-t
propor-tions so as to give four different loadings of china
clay in -the final dry paper. The resul-ting mixtures were
run through a conduit 13 to the head box 14 of a Four-
' drini0r paper making machine 15 where, :~or each loading
i of clay, a veb of paper was ~ormed on the wire, dewatered
and thermally dried.
Samples of the paper web for each loading of
çlay were weighed dry and then incinerated and the weight
of ash was used to calculate the percentage by weight of
clay in the dry paper, after allowing for the loss on
ignition of the clay.
Other samples of each paper web were tested
` 25 for burst strength by the test prescribed in TAPPI
Standard T403-os-74~ the burst strength being defined
as the hydrostatic pressure, in kilonewtons per square
metre, required to produce rupture of the material when
~ the pressure is increased at a controlled constant rate
through a rubber dia7hragm to a circular area 30.5 mm in
-- 7 --
~7~67
diameter with the area of the material under test being
initially flat and held rigidly at the circumference bu-t
free to bulge during the test.
B. A second batch of paper samples was prepared
in a manner similar to that described at A above except
that the cationic starch was mixed with the suspension of
fibres and with the size and aluminium sulphate in stirred
tank 1 and not with the~fi~ler in tank 11. The amount of
starch used was 2% by weight based on the weight of dry
cellulosic Eibres. The suspension was diluted with water
in tank 10, as in A, and dif:Eeren-t quan-tities of an aqueous
suspension of the same china clay filler were added to give
four difEerent loadings o:E the clay f:illler. A web of paper
was formed Eor each loacling of clay filler and measurements
of the percentage by we:ight o:E clay in the dry paper and of
the burst strength were made.
C. A third batch of paper samples was prepared
in a manner similar to that described at A above except
that the china clay -fill-er was mixed with the suspension
o~ fibres and with the size and aluminium sulphate in
stirred tank 1. Again the quantities of china clay filler
used were varied to give four different loadings o-f clay
in the final paper. The suspension was diluted with water
in tank 10, as in A, and a solution of the cationic starch
was run in from stirred tank 11 in a quantity su~ficient to
provide 5% by weight of starch based on the weight o-f clay,
A web o:E paper was formed for each loading of clay and
measurements of the percentage by weight o-f clay in the
dry paper and of the burst strength were made.
D A fourth batch~ of paper samples was prepared
~74~7
in a manner similar to that described at A above except
tha-t no tertiary cationic starch was added. The suspension
of fibres, size and aluminium sulphate were mixed in stirred
tank 1 and the mixture was diluted with water in tank 10,
as in A, and again different quantities of china clay
,l filler were added in tank 10 to glve four different loadingsof the ~ay in -the final paper. A web of paper was formed
for each loading of clay and measurements of the percentage
by weight of clay in the dry paper and of the burst strength
were made.
The results o:E Tes-ts, ~, B, C and D are set
forth in Table 1 below. The burst strength figures were
expressed as a percentage of the burst strength of a sized
paper web which conained no fi].ler and no starch and the
resultant relative burst strengths were plotted graphically
against the p~rcentage by weight of clay in the web. From
the graphs thus obtained the relative burst strengths
corresponding to clay filler loadings of 10%, 17.5% and
25% by weight were found for each batch of paper. Table 1
also give the percentage by weight of cationic starch
based on the weight of dry furnish (total weight of clay
and fibres~ for each web of paper.
._ _ . _ .. . . . . . _ _ . _ ._.. A . '
~ 7467 ::
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The results show that, especially at high loadings,
mixing the cationic starch with the clay filler and then adding
the mixture containing flocs or starch and clay filler to the
suspension of sized cellulosic fibres gives an unexpectedly
high strength value for the resultant paper for a given weight
of cationic starch per 100 g of dry urnish.
EXAMPLE 2
Further batches of paper were made according ~o
the method described in Example lA, (using the same
apparatus) except that the proportion of ca~ionic starch
mixed with the china clay in stirred tank 11 was varied for
each batch, the proportions of starch being 5%, 7.5~, 10%,
15~ and 20% by weight, respectively, based on the weight of
dry clay. For each proportion of starch to clay, webs of
paper were formed containing three different loadings of
starch-treated clay filler. Samples of each web were tested
for burst strength and the perCentacJe of clay filler in
the dry paper. The results were plotted graphically and
the relative burst strength for a loading of 20% by ~eight
of dry clay based on the weight of dry fibres was found
for each batch of paper. The results obtained are set forth
in Table II below.
TABLE II
% by weight of % by weight of Relative burs-t
starch on clay starch on furnish strength for a clay
filler loading o~
20% bv wt.
1.0 74
7.5 1 r 5 77
2.0 79
3.0 82
4.0 84
~7~67
It can be seen from these results that further
improvements in the strength of the paper can be achieved
by raising the proportion of starch but that the improvements
become smaller as the proportion of starch is increased.
Also when the proportion of starch was 20% by weight, based
on the weight of clay, some starch was found in the "white
water" i.e. the water which passed through the wire of the
Fourdrinier paper making machine.
EXAMPL~ 3
A further batch of paper was made by adding 2.5%
by weight of the cationic starch contalning tertiary amine
groups, hased on the weight of dry fibres, to the suspension
of cellulosic fibres and the si~e and aluminium sulphate
was
in stirred tank 1. In tank 10 there ~crc mixed with the
suspension o:E -treated fibres an a~ueous suspension of the
china clay :Eiller which had heen treated with a :Eurther
5% by weight of starch based on the weight of clay. The
resultant ~i..tu~ was formed into paper on -the ~ourdrinier
paper making machine 15 and the percentage by weight of clay
in the dry paper and the relati.ve hurst strength were
determined. The percentage by weight of clay in the paper
was 27% and for every 100 g of dry furnish (clay and
cellulosic fibres~ there were present 1.36 g of starch
- associates with the fibres and 1.35 g of starch associated
wi-th the clay filler, making a total of 2.71 g. The relative
burst strength of the paper was 88%.
By comparison: (i) a paper containing the same
percentage by weight of clay but prepared by the method
of ~xample 1 A (1.35 g of starch per 100 g of dry furnish)
had a relative burst strength of 63%; (ii) a paper
containing the same percentage by weight of clay but
~L~97~67
prepared by the method of E~ample lB (1.46 g of starch
per 100 g of dry furnish) had a relative burst strength
of 61%; (iii) a paper containing the same percentage by
weight of clay but prepared by the method of Example lD
(no starch) had a relative burst stren~th of 38%; and
(iv) a paper con-tainin~ the same percenta~e b~ wei~ht of
cla~ and prepared b~ the method o:E Example lA but with a
~reater proportion of starch (2.80 g of starch per 100
of dry furnish~ had a relative burst strer,gth of 68%.
EXAMPLE _
An aqueous suspension containing 2% by weigh-t
of cellulosic fibres obtained by heating and re:Eining a
bleached sulphite pulp was mixed in a stirred tank with
1.5% by weight, based on the weight o:E dry Eibres, of
:Eortified rosin size and 3.0% by weigh-t of powdered
aluminium sulphate. The suspension of sized Elbres was
then passed to a second tank where the suspension was
mi~ed with three times its own weight of water to dilute
the suspension to 0.5% by weight of fibres.
In a thi.rd stirred tank there was mixed together
water, a china clay filler A in a floccula-ted state, and
a cationic starch.(China clay filler A had a particle
size distribution such tha-t 31% by weight consisted of
particles having an equivalent spherical diameter (e.s.d)
25 j larger than lO~m, 13% by weight consisted of particles
having an e.s.d. smaller than ~ and 7% by weight
consisted of particles having an e.s.d. smaller than l~m).
W~S
The starch-~ added in the proportion 5% by weight,
based on the weight of dry clay.
The flocculated mixture of clay filler A and
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7~67
starch was run -to a further tank where lt was mixed with
the suspension ~f sized celluiosic fibres in a given
proportion so as to give a particular loading of china
clay filler in the final dry paper. The resultant mixture
was then passed to the head-box of a Fourdrinier paper
making machine on which a web of paper was formed on the
wire, dewatered and thermally dried. Further mixtures of
china clay and starch and sized fibres in different
proportions were prepared in a similar manner and formed
into paper webs, dewatered and dried. Samples of the paper
web for each loading of clay were weighed dry and then
incinerated and the weight of ash was used to calculate
the percentage by weight of clay in the dry paper, after
allowing for the loss on ignition of -the clay. Other
samples of each paper were tested Eor burs-t strength by
the test prescribed in TAPPI Standard T~03-Os-7~.
A fur-ther series of similar experiments was
perEormed using a different china clay filler B which
had a particle size distribution such that 25% by weight
consisted of particles having an equivalent spherical
diameter larger than 10~m, 23% by weight consisted of
particles having an equivalent spherical diameter smaller
than 2~m and 1~% by weight consisted of particles having
an equivalent spherical diameter smaller than l~m. Filler
B was mixed with 5% by weight, based on the weight of dry
clay, of the same cationic starch in the same manner as
described above.
Further series of experiments were performed
using China clay Fillers A and B but no -tertiary cationic
starch. Aqueous suspensions of the two fillers were mixed
a74~7
directly with a suspension of fibres, rosin size and
aluminium sulphate and webs of paper were formed and tested
as above.
In each case the percentage by weight of filler
in the filled paper was plotted against the burst ratio
of the filled paper expressed as a percentage of the burst
ratio for a sheet of paper prepared from the same fibre
stock but containing no filler. The burst ratio is the
burst strength divlded by the weight per unit area of
the paper. The percentage burst ratios corresponding to
filler loadings of 10%, 15%, 20%, 25%, and 307~ by weight
were -then read from the graph for each series of
experiments.
; The results obtained are set forth in the Table
III below.
467
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7~67
These results show that not only do the fillers
which have been treated with the cationlc starch before
mixing with the cellulosic fibres give papers of
considerably higher burst strength as compared with papers
containing equivalent quantities of the untreated fillers
but that a treated china clay filler containing a small
proportion of fine particles gives a further substantial
and unexpected improvement in strength as compared with
a treated conventional china clay filler.
- EXAMPLE 5
An aqueous suspension containing 0.5% by weight -
o-f sized cellulosic fibres derived from bleached sulphite
pulp was prepared as described in Example 1. ~ater, kaolin
clay filler in a flocculated state and a cationic starch
containing tertiary amine groups were mixed together in a
vessel of internal diameter 10 inches which was provided
with a propeller turbine of overall diameter 5 inches.
The clay and cationic starch were the same as those used
in Example 1 and the starch was added in the proportion 5%
b-~ weight, based on the weight of dry clay. The turbine was
run for 5 minutes at a speed of 1500 r.p.m. and it was
found that the amount of shear thus provi~ed was sufficient
to ensure that substantially all of the mixture passed
through a No. 200 mesh British Standard sieve. The
flocculated mixture was then mixed with the suspension of
sized fibres in different proportions so as to ~ive five
different loadings of clay filler in the final dry paper,
care being taken to ensure that the shear applied to the
mixture was no more severe than that exerted during the
preparation of the clay/starch mixture. ~'or each loading ~'
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741Ei7
of clay a web of paper was formed on the wire of the
Fourdrinier paper making machine, dewatered and thermally
dried. Samples of the web for each loading of clay filler
were then tested for percentage by weight of clay in the
dry paper and for burst strength as described in Example 1.
The experiment was then repea-ted except that the
clay and ca-tionic starch were mixed by hand stirring so that
~inimal shear was applied and the suspension of sized
fibres was mixed with the clay/starch mixture in a similar-
manner. When an a-ttempt was made to pour the aqueous clay/
starch mixturelthrough a No. 2bo mesh British S-tandard
sieve it was found tha-t a considerable proportion was
re-tained in the sieve. The webs oi~ paper formed from the
mixture w~ere found on visual inspection -to be unacceptable
on account o:E the nonuni:Eormity of the paper due to l~ps
of undispersed fi.ller.
The experiment was repeated again except that
the clay and cationic starch were mixed by means of the
propel].er turbine for 5 minutes but at a speed of 7000
r.p.m. The resultant mixture passed not only through a
No. 200 mesh British Standard sieve but also substantially
completely through a No. 300 mesh British Standard sieve
(nominal aperture 53~m~ and it was clear that the clay/
starch mixture was little, if any, coarser than the
untreated clay filler. For each loading of clay a web of
paper was formed on the wire of the Fourdrinnier paper
making machine, dewatered and thermally dried. Samples of
the web for each loading of clay were then tested for
percentage by weigh-t of clay in the dry paper and for
- 30 burst strength.
.
- 18 -
746Y7
Finally, as a control, the experiment was
repeated again except that no cationic starch was added.
For each loading of clay~ a web o~ paper was formed on the
wire of the Eourdrinnier paper making machine, dewatered
and thermally dried. Samples of the web for each loading of
clay were then tested for percentage by weight of clay in
the dry paper and ~our burst strength.
The results obtained are set forth in Table IV
below. In each case the burst strength figures~Y~re expressed
as a percentage o-f the burst strength of a sized paper
web which contained no filler and no starch and the resultant
relative burst strengths were plotted graphically against
the percentage by weight of clay in the web. From the
resultant graphs the relative burst strengths corresponding
to loadings of 5%, 10%, 15%, 20C,~o and 25% by weight of
clay were found for each batch of paper.
TABLE IV
Clay loading wt.% 5 lo 15 20 25
Relative burst strengths
Low shear Paper unacceptable
Moderate shear 95 89 83 77 70
High shear 94 87 80 71 62
No starch 84 71 61 51 42
r
,
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